1. Field of the Invention
The present invention relates to a television camera using a solid state image sensor such as a CCD (charge coupled device), and is directed more particularly to a smear reduction system for the solid state television camera.
2 Description of the Prior Art
Recently, solid state imaging devices have been developed in many laboratories in accordance with the advancement of semiconductor technology. Among various types of solid state image sensors, a charge coupled device (hereinafter, simply referred to as CCD) imager is of particular interest.
There are two types of the CCD imagers separated by the arrangement of the CCD. One is of a so-called frame-transfer type and another is of an interline-transfer type. The former type of the CCD imager has an image sensing array composed of a plurality of individual light sensing units arranged in both horizontal and vertical lines, temporary storage devices of the same number as that of the light sensing units for storing and transferring the charge accumulated in the light sensing units and a shift register for receiving the charge sequentially from the storage devices and generating a signal representing the image. In this type of the CCD imager, as the light sensing units are irradiated by the light of an image during the period of the transfer of the charge from the image sensing array to the temporary storage area, an undesirable charge is mixed to the transferred charge representing the image during this transfer period. That may cause an undesirable vertical bright line in a reproduced picture, which is a so-called "smear". The interline-transfer type CCD imager has a gate circuit between each light sensing unit and each charge transfer device, and this gate circuit is turned off during the transfer period, so that leakages of the charges due to the reasons described above in the frame-transfer type CCD imager can be prevented. However, in this type of the CCD imager, as the light sensing units, the gate circuits and the charge transfer devices are all formed in the same bulk, the light of the bright image with a long wave length enters into the deep layer of the bulk from the light sensing units and generates the charge in that area, and this charge spreads horizontally and leaks to the portion of the charge transfer devices. This causes a smear as described above.
The existing system for reducing the smear is available by masking several horizontal lines of the light sensing units, detecting the output signal corresponding to the masked lines and subtracting the detected output from the output of the image sensing device. This is, for example, shown in U.S. Pat. No. 4,010,319. But, the defect of the above system is that additional elements, namely the masks, are required, and hence the masking process is further required thus causing the manufacturing cost to be increased.